docs: def qop(self):
          """Indicates what "quality of protection" the client has applied to
          the message for HTTP digest auth."""
          def on_update(header_set):
              if not header_set and 'qop' in self:
                  del self['qop']
              elif header_set:
                  self['qop'] = header_set.to_header()
          return parse_set_header(self.get('qop'), on_update)